Sometime before the Target(TGT) hack, Experian had its own data leak -- via a subsidiary. That data leak got plugged before Target sent victims to Experian. But it shows that even those entrusted with our most sensitive data don't know how to protect it.

Experian unknowingly sold the personal data of millions of Americans -- including Social Security numbers -- to a fraudster in Vietnam. That guy then sold the personal information to identity thieves around the globe.

It wasn't until U.S. Secret Service agents alerted Experian that the company stopped.

Hieu Minh Ngo, now 25, was caught and admitted to posing as a private investigator in Singapore to get exclusive access to data via Court Ventures, an Experian subsidiary. Ngo then sold access to fellow criminals.

It's been more than a year since Experian was notified of the leak. Yet the company still won't say how many Americans were affected.

CNNMoney asked Experian to detail the scope of the breach. The company refused.

"As we've said consistently, it is an unfortunate and isolated issue -- one that did not affect Experian's databases and has no true relevance to the work we did with clients like Target," Experian spokesman Gerry Tschopp said.

Federal court filings show that at least one database actually belonged to another firm -- U.S. Info Search. It was Experian's subsidiary that sold database access to Ngo.